The present invention generally relates to semiconductor devices and more particularly to a semiconductor device having a region doped with impurity to such a level that the concentration of the impurity exceeds the solubility limit of the region.
With ever increasing demand to improve the operational speed of transistors, efforts are made to manufacture a base region of bipolar transistor as thin as possible. By doing so, one can reduce the time for a carrier to move across the base region by diffusion. However, such a decrease in the thickness of the base region leads to an increase in the spreading resistance in lateral directions.
In order to cancel or compensate the effect of this increased spreading resistance, it is desired to increase the concentration of the impurities in the base region so that the resistance thereof is decreased. However, such an increase in the impurity in the base region has to be accompanied with corresponding increase in the impurity level in a corresponding emitter region which has an impurity level much higher than the impurity level of the base region. The impurity level of the emitter region is chosen such that the carrier density in the base region maintains an optimum proportion or ratio with respect to the carrier density in the emitter region.
When the impurity level in the emitter region becomes excessive, the emitter region is saturated with the impurity and there appears a precipitation of the doped impurity as separate phase. Thus, the impurity level in the base region is limited. When such a precipitate appears in the emitter region, the increase in the impurity or dopant does not contribute to the increase of the carrier density any more, and the proportion of the carrier density in the emitter region to that in the base region is deviated from the optimum ratio when the impurity level of the base region is increased. Further, such an existence of the precipitates in a host silicon crystal causes a scattering of carrier in the silicon and facilitates its recombination. Thus, such an increase in the impurity level in the base region is conventionally thought as a disadvantage.
For example, a conventional npn bipolar transistor has a base region having a thickness of about 3000 .ANG. and the base region is doped with boron with a level of 5.times.10.sup.17 -1.times.10.sup.18 /cm.sup.3. The transistor also has an emitter region doped with arsenic with a level of about 1.times.10.sup.20 /cm.sup.3. When the thickness of the base region is reduced to about 1000-1500 .ANG., it is desired that the impurity level in the base region is increased to about 1.times.10.sup.19 /cm.sup.3 . As it is necessary to maintain the impurity level of the emitter region larger than that of the base region with a factor of about 1000, the corresponding arsenic concentration level in the emitter region should be in the order of 10.sup.22 /cm.sup.3. However, this level of the emitter region exceeds the solubility limit of arsenic in the silicon crystal at 1000.degree. C. The value of the solubility limit is about 4.times.10.sup.21 /cm.sup.3. Thus, the precipitates appears as already described when the emitter region is doped to such an impurity level. In order to avoid the various disadvantages accompanying the appearance of the precipitate, one has to limit the impurity level of the emitter region to a value substantially smaller than the solubility limit. However, by limiting the impurity level of the emitter region as such, the ratio of the carrier density in the emitter region to that of the base region is reduced to below 1000. With such a small difference in the carrier concentration level in the emitter and the base, the common emitter current gain of the transistor is expected to be reduced to lower than about 100. The excessive increase in the impurity level in the emitter also invites contraction of the band gap in emitter and such a contraction of the band gap also leads to the decrease of the common emitter current gain.
Meanwhile, semiconductor devices of various type are constructed on a silicon wafer, and such a semiconductor device uses doped single crystal or polycrystal silicon as a conductive region. For example, such a doped silicon is used as a gate electrode of a MOS transistor or a bit line of memory device. In a bipolar transistor, too, such a doped silicon conductive region is used as electrodes as well as a diffusion source layer which releases the impurity into the base region to form a shallow emitter region in the base.
Such devices are required to exhibit improved response or operational speed, and for this purpose, it is necessary to lower the resistivity of such a conductive region as much as possible so as to reduce the time constant of input and output signal path of the device formed by such a conductive region. For this purpose, it is desired to dope the impurity as much as possible. However, when the silicon is doped with impurity by diffusion from a source material contacting with the silicon as is practised conventionally, the level of the impurity to be introduced into the silicon is limited by the thermodynamic equilibrium at the boundary of the silicon and the source material. In other words, there is an upper limit in the level of impurity to be introduced into the silicon, and corresponding thereto, there is a lower limit in the resistivity of silicon. In the case of a polysilicon film having a thickness of about 4000 .ANG., the lowest possible surface resistivity is about 10 .OMEGA./.quadrature..
Conventionally, various silicides are also used for the conductive region in order to achieve low resistivity. However, the formation of silicide requires heat treatment for a substantial time period and there is a substantial risk that such a heat treatment deteriorates the profile of the impurity distribution in the semiconductor device. Further, the silicides tend to be detached from the silicon substrate underneath. For example, the tungsten silicide requires heat treatment at 900.degree. C. for a substantial period of time and is easily detached from the substrate.